"I usually shoot my Glock 17 and I might start out strong, landing hits in a 2.5x2.5" post-it note at 7-10 yards but after awhile, shots start going low and left."

This tells me that you are developing a "flinch". Stop shooting while you are doing good, take a break, come back to it later. I also notice this if my time/rounds shot becomes too long. The low/left also indicates "flinch" or anticipating the shot.